[Monotypic IgM euglobulin precipitating upon exposure to air at room temperature].

A monoclonal lambda IgM euglobulin with peculiar precipitable characteristics in vitro was found in the serum of a patient with Waldenström macroglobulinaemia. The precipitation was observed at room temperature only when serum was exposed to air. Covalent disulphide bonds probably contribute to the mechanism involved in the formation of precipitate in this case.